Interpretive Summary: Spodoptera frugiperda, fall armyworm (FAW) is an important polyphagous agricultural pest. In the U.S., FAW commonly infests maize, cotton, bermudagrass, peanuts, and sorghum, but its host range includes plants from 68 genera. Although this insect is controlled by a wide range of synthetic and biological insecticides, there is variability in its susceptibility, especially to the soil bacterium Bacillus thuringiensis. Some populations of FAW have developed resistance to the toxin produced by B. thuringensis. We have determined the mortality values of FAW populations either collected in the wild or maintained in the laboratory by exposing to a series of dilutions of recombinant purified proteins, Cry1F and Cry1Ac produced from Pseudomonas fluorescenson. A total of 137 isofamilies: 22 from Mexico (Tamaulipas and New Mexico), 35 from United States (Mississippi and Texas), and 80 from Puerto Rico (Santa Isabela) were evaluated. Results indicate that some isofimilies of S. frugiperda from Puerto Rico had significant differences in resistance ratios when compared with a susceptible colony. However, the measured tolerance to Cry1F and Cry1Ac may not be stable. In order to keep the resistant alleles at high frequency in the colonies it is important to continue the selection throughout consecutive generations.

Technical Abstract:
The fall armyworm (Spodoptera frugiperda), is one of the most important pests of the American continent. Its control has relied primarily on multiple applications of insecticides that can amount up to 1,000 grams of active ingredient per hectare on some of the approximately 30 crops that this insect damages. The use of genetically engineered crops that express Bacillus thuringiensis (Bt) toxins, Bt-corn and Bt-cotton, are other means of controlling this insect. However, S. frugiperda is one of the least susceptible Lepidoptera species to Bt proteins, and a case of high tolerance to Bt-corn has already being reported. We found that the susceptibility to Cry1Ac and Cry1Fa proteins of Bt in 133 isofamilies from 5 regions of three countries was similar to the susceptibility of two Bt-susceptible laboratory colonies to these proteins. Four Puerto Rico isofamilies had high tolerance to Cry1Fa and not-so-elevated tolerance to Cry1Ac. Two of these four isofamilies were backcrossed with a Bt-susceptible laboratory colony and their progeny was as susceptible to both Bt proteins, as the Bt-susceptible colony, indicating that Bt-resistance is a recessive trait in these samples.